The present invention relates to an intermediate transfer unit used in an image formation apparatus using an electrophotographic method, such as a copying machine, a printer, and a facsimile. The present invention also relates to a recording medium carrier system applied to the image formation apparatus.
As for a copying machine, a printer, a facsimile and other image formation apparatuses respectively using electrophotography, primarily an image formation apparatus using a laser beam writing device, it is important to fix a toner image while carrying a recording medium at high speed in order to make good use of the apparatus. It is also important to provide a simple means for relieving a paper jam or other problems caused by such operation.
Generally, an image formation apparatus using electrophotographic technology is provided with a photoconductive drum having a photosensitive layer as the peripheral face, charge means for evenly charging the peripheral surface of the photoconductive drum, exposure means for selectively exposing the evenly charged peripheral surface to form an electrostatic latent image, developing means for applying toner as a developer to the electrostatic latent image formed by the exposure means to form a visible image (a toner image), and transfer means for transferring the toner image developed by the developing means to a transfer medium such as paper.
For transfer means for transferring a toner image developed on a photoconductive drum on a transfer medium, such as paper, heretofore, there is known transfer means provided with an intermediate transfer belt to which a toner image formed on a photoconductive drum is transferred (primary transfer) and which further transfers (secondary transfer) the toner image onto a recording medium, and with a driving roller for circulating the intermediate transfer belt.
As for the above prior transfer means, there is a problem that since a distance between a primary transfer position and the driving roller is large, the amount of shrinkage of the intermediate transfer belt between the primary transfer position and the driving roller is increased and the travel speed of the intermediate transfer belt in the primary transfer position is unstable. As a result, it is difficult to acquire satisfactory primary transfer.
Further, according to the above prior transfer means, there is a problem that a transfer roller directly touches the joint of the intermediate transfer belt, staining a secondary transfer roller by toner accumulated in a step of the joint of the intermediate transfer belt, and causing toner to adhere to the rear of a recording medium in a subsequent secondary transfer.
Further, according to the above prior transfer means, there is a problem that when a thin line image is transferred onto a recording medium, the surface of which is smooth, a failure of the transfer of toner (a void) occurs.
Further, according to the above prior transfer means, there is a problem that even if transfer on a recording medium having a smooth surface is satisfactory, transfer on a recording medium having a rough surface is insufficient. Particularly, when multiple layers of toner are transferred as a multiple color image, a failure to transfer toner far from the surface of a recording medium occurs.
Further, according to the above prior transfer means, there is a problem that in primary or secondary transfer, the deterioration of transfer efficiency and the omission (void) of a part of a toner image in transfer occurs. Also, in secondary transfer, there is a problem that it is difficult to transfer on a recording medium the surface of which is extremely irregular, such as recycled paper and bond paper, without lacking a part of an image. There is also a problem that if toner having a high fluidity is used, toner is readily scattered in transfer. In particular, if a primary or secondary transfer means which functions as a transfer electrode for applying transfer voltage to a transfer position, is located in a position distant from its transfer position, a transfer electric field in the transfer position cannot be concentrated upon the transfer position, and a toner image is scattered due to electrostatic force. Also, if for example, if the intermediate transfer belt is wound on the photoconductive drum without means for substantially pressing the intermediate transfer belt on the photoconductive drum or a recording medium in a transfer position, the area in which the photoconductive drum and the intermediate transfer belt are in contact in a transfer position is large and the turbulence of a toner image due to mechanical force caused by slight difference in speed between both and others readily occurs.
Further, according to the above prior transfer means, a monolayer or multilayer belt in which a conductive, a semiconductive or an insulating resin layer is generally formed, at least as the surface layer, is used for the intermediate transfer belt. Thus, there is a problem that, since the surface is made of resin as described above, friction and scratches are readily generated. In particular, a large quantity of particulates of metallic oxide generally adhere to the surface of a toner particle as an additive and, since the above additive is much harder than resin constituting the surface of the intermediate transfer belt, it is readily embedded in the intermediate transfer belt. Further, a phenomenon (so-called filming) in which toner adheres to the intermediate transfer belt in the embedded point, mentioned above, occurs and deteriorates the image. For example, the transfer efficiency in primary or secondary transfer deteriorates and a void (i.e., the lack of a part of a toner image in transfer) occurs. Also, in secondary transfer, there is a problem that it is difficult to transfer on a recording medium having a surface that is extremely irregular, such as recycled paper and bond paper, without causing an imperfection in an image.
Further, according to the above prior transfer means, there is a problem that a void occurs in a part of a toner image transferred on the intermediate transfer belt in primary transfer, particularly the center. Also, in secondary transfer, there is a problem that it is difficult to transfer on a recording medium having an extremely irregular surface, such as recycled paper and bond paper, without causing an imperfect image, in addition to the above problem of a void.
Further, in an image formation apparatus for forming a full color image by overlapping plural colors, for example, the secondary transfer means is prevented from being stained by controlling the driving of the secondary transfer means for executing secondary transfer so that the secondary transfer means is not in contact with the intermediate transfer belt while images of each color are formed. Instead, the secondary transfer means is touched to the intermediate transfer belt after the final image is formed and, when secondary transfer is started after primary transfer is finished, an image on the intermediate transfer belt is not disturbed. However, there is a problem that when the intermediate transfer belt is vibrated, such as when the secondary transfer means is switched to a state in contact or not in contact with the intermediate transfer belt, the speed is varied and turbulence of an image occurs.
Further, according to the above prior transfer means, transferability in a primary transfer part is insufficient. Concretely, there are problems in the quantity of toner (the thickness of the layer), dispersion in resistance among each member, the variation of transfer efficiency due to the variation of resistance, a phenomenon of a void, and the stability of the density due to aging.
Further, according to the above prior transfer means, transferability in a secondary transfer part is insufficient. Concretely, there are problems in, the quantity of toner (the thickness of the layer), the type of a recording medium such as plain paper, a postal card, and OHP sheet, dispersion in resistance and the variation of resistance among each member, the variation of transfer efficiency due to the variation of resistance by environment, a phenomenon of a void, and the stability of the density due to aging.
Further, with respect to resistance, which is an important characteristic of a primary transfer member and a secondary transfer member, the above transfer means includes members having approximately the same variation of resistance due to environment are used for both the primary and secondary transfer members. Therefore, if members having a small variation of resistance due to environment are used for both primary and secondary transfer members, current may leak in a part not related to transfer and the failure of transfer may occur, particularly in a case where a recording medium, such as a postal card or an envelope smaller in size than the width of the secondary transfer member, is printed in an environment of low temperature and low humidity in which the resistance of the recording medium is higher than that of the secondary transfer member in a secondary transfer part. To avoid the above situation, it is possible to increase the resistance of the secondary transfer member and reduce leakage current. However, since a member having small variation of resistance due to environment generally has a large dispersion of the resistance, there is a problem that the nonuniformity of transfer partly occurs.
In the meantime, if members having large variation of resistance due to environment is used for both primary and secondary transfer members, no failure due to a leak of secondary transfer occurs because the resistance of the secondary transfer member changes approximately as the change of the resistance of a recording medium due to environment. However, voltage required in a primary transfer part in the environment of low temperature and low humidity causes the cost to increase.
Further, in a prior transfer means as disclosed in Japanese Patent Application No. Hei. 7-322667, an imperfect image is prevented from occurring at the simultaneous timing of primary transfer and secondary transfer by providing a conductive layer on the intermediate transfer belt and setting a relationship between resistance RT of a part from a primary transfer bias applying power source to the conductive layer and apparent resistance R1 in a primary transfer part so that RT less than R1.
According to above prior transfer means, it is difficult, depending upon environment and the type of paper, to prevent an imperfect image from occurring at the simultaneous timing of primary transfer and secondary transfer. Concretely, if current which flows in a secondary transfer is larger than current which flows in a primary transfer, the phenomenon is remarkable.
The present invention is made to solve the above problems, and an object thereof is to provide a recording medium carrier system which is capable of easily dealing with various troubles caused by high-speed carriage of recording paper.
Another object of the invention is to provide an intermediate transfer unit by which the travel speed of an intermediate transfer belt in a primary transfer position can be stabilized.
Still another object of the invention is to provide an intermediate transfer unit by which the rear of a recording medium is not stained using an intermediate transfer belt with a joint.
Still another object of the invention is to provide an intermediate transfer unit for enabling satisfactory transfer onto a recording medium having a smooth surface such as OHP or having a rough surface, such as bond paper. The object is also to provide an intermediate transfer unit for enabling satisfactory transfer onto a recording medium the surface of which is smooth, in an overall area in the direction of the shaft of a transfer roller. The object is also to provide a compact and low-cost intermediate transfer unit for enabling satisfactory transfer onto a recording medium having a rough surface and simultaneously for enabling the reduction of torque for driving a transfer roller. The object is further to provide an intermediate transfer unit for enabling satisfactory transfer onto a recording medium having either a rough or a smooth surface while simultaneously maintaining a high quality image over long term use. The object is furthermore to provide an intermediate transfer unit for enabling the formation of an image approximately uniform in color in any density area on a recording medium having either a rough or smooth surface.
Still another object of the invention is to provide an intermediate transfer unit for forming a satisfactory image, without the lack of a part of an image such-as a void in transfer.
Still another object of the invention is to provide an intermediate transfer unit enabling the stabilization of transferability (transfer efficiency) in a primary transfer part.
Still another object of the invention is to provide an intermediate transfer unit enabling the stabilization of transferability (transfer efficiency) in the secondary transfer part.
Still another object of the invention is to provide an intermediate transfer unit enabling the stabilization of transferability (transfer efficiency) in the secondary transfer part and the reduction of the capacity of the high-voltage power source.
Still another object of the invention is to provide an intermediate transfer unit which can prevent the deterioration of an image in simultaneous transfer of primary transfer and secondary transfer.
In order to achieve the above objects, according to a first aspect of the invention, in a recording medium carrier system, a paper feed mechanism for carrying a recording medium to a transfer part, a mechanism for transferring a toner image onto a recording medium, a mechanism for fixing the transferred toner image on the recording medium, and a mechanism for ejecting the recording medium from a fixing part are respectively constituted as independent units.
According to a second aspect of the invention, an intermediate transfer unit is provided with an intermediate transfer belt to which a toner image formed on a photoconductive drum is primarily transferred and which further secondarily transfers the toner image onto a recording medium, and with a driving roller for circulating the intermediate transfer belt and is characterized in that the above primary transfer position is arranged close to the driving roller.
According to the intermediate transfer unit of the second aspect, since the primary transfer position is arranged close to the driving roller, the shrinkage of the intermediate transfer belt between the primary transfer position and the driving roller is reduced, the travelling speed of the intermediate transfer belt in the primary transfer position is stable and, as a result, primary transfer in a satisfactory state is readily acquired.
According to a third aspect of the invention, an intermediate transfer unit is provided with an intermediate transfer belt with a joint to which a toner image formed on a photoconductive drum is primarily transferred by a primary transfer member and which further secondarily transfers the toner image onto a recording medium using a secondary transfer roller, and with a driving roller for circulating the intermediate transfer belt and is characterized in that an electric field in a direction in which the above toner is returned from the secondary transfer roller to the intermediate transfer belt is formed while the secondary transfer roller is pressed on the intermediate transfer belt when no image is formed, and the secondary transfer roller is detached when the joint of the intermediate transfer belt is opposite to the secondary transfer roller.
According to the intermediate transfer unit of the third aspect, it is possible to prevent toner from adhering to the secondary transfer roller due to direct contact thereof with the joint of the intermediate transfer medium. Therefore, the rear of a recording medium will not be stained enabling the intermediate transfer unit to satisfactory transfer the toner.
A fourth aspect of the invention includes an intermediate transfer unit provided with an intermediate transfer belt which receives toner image formed on a photoconductive drum and transferred by a primary transfer member and which secondarily transfers the toner image onto a recording medium using a secondary transfer roller, wherein a driving roller drives the intermediate transfer belt. The fourth aspect of the invention is characterized in that the intermediate transfer belt includes dispersed fluoric particulates, at least in the surface layer, and the secondary transfer roller is pressed on the intermediate transfer belt under the linear pressure of 27 gf/mm or less.
Also, in the above intermediate transfer unit, the hardness of the above secondary transfer roller is set to 70xc2x0 or less, as measured by a Asker-C hardness meter.
Also, in the above intermediate transfer unit, plural types of additives different in a particle diameter are added in the above toner and the surface coverage of them is 2 or more.
Also, in the above intermediate transfer unit, the above toner image transferred on the above intermediate transfer belt is 1.5 mg/cm2 or less per unit area in any density area.
According to the intermediate transfer unit of the fourth aspect of the invention, since the intermediate transfer belt has an excellent mold releasing property, toner is readily released in secondary transfer, and when a thin line image is transferred onto a recording medium having a smooth surface satisfactory transfer is enabled even if pressure applied to the toner is not fixed. Further, since the hardness of the secondary transfer roller is set to 70xc2x0 or less, as measured by Asker-C hardness meter, the concentration of transfer pressure is avoided in a linear image on the intermediate transfer belt and the occurrence of a void can be reduced.
Also, according to the above intermediate transfer unit, since pressure applied to the toner is uniform when a thin line image is transferred onto a recording medium having a smooth surface, satisfactory transfer is enabled.
Also, according to the above intermediate transfer unit, since an additive with a relatively large particle diameter is added, the additive is not embedded in a mother particle for a long term but the fluidity is maintained and the quality of an image is stable, and since an additive with a relatively small particle diameter is added, the surface coverage is large compared with the added weight, and even if pressure applied to toner is not fixed when a thin line image is transferred onto a recording medium having a smooth surface satisfactory transfer is enabled.
Also, according to the above intermediate transfer unit, since the height of a toner layer is limited and pressure applied to toner is made uniform when a thin line image is transferred onto a smooth recording surface by forming a toner layer in any density area under the condition that the quantity of toner to be transferred secondarily is 1.5 mg/cm2 or less, satisfactory transfer is enabled.
A fifth aspect of the invention includes an intermediate transfer unit provided with an intermediate transfer belt which receives a toner image formed on a photoconductive drum and transferred by a primary transfer member and which secondarily transfers the toner image onto a recording medium using a secondary transfer roller, wherein a driving roller drives the intermediate transfer belt. The fifth aspect of the present invention is characterized in that the toner is coated with an additive at the surface coverage of 2 or more and the above secondary transfer roller is pressed on the intermediate transfer belt under the linear pressure of 15 gf/mm or more.
Also, in the above intermediate transfer unit, the hardness of the above secondary transfer roller is set to 50xc2x0 or more, as measured by Asker-C hardness meter.
Also, in the above intermediate transfer unit, plural types of additives different in a particle diameter are added in the above toner.
Also, in the above intermediate transfer unit, the toner image transferred on the intermediate transfer belt is 1.5 mg/cm2 or less per unit area in any density area.
According to the intermediate transfer unit of the fifth aspect of the invention, since toner is coated with a sufficient quantity of additive, the force of the toner which adheres to the intermediate transfer belt can be reduced, toner can be also transferred in a concave portion of a recording medium the surface of which is rough, and secondary transfer in a satisfactory state can be readily acquired. Further, since a recording medium having a rough surface is pressed on the intermediate transfer belt under sufficient linear pressure, the concave portion of the recording medium can be brought close to a toner image on the intermediate transfer belt, and secondary transfer in a satisfactory state can be readily acquired.
Also, according to the above intermediate transfer unit, since the increase of driving torque by the excessive broadening of a secondary transfer nip formed by the secondary transfer roller and the intermediate transfer belt can be prevented, a driving motor can be miniaturized and an intermediate transfer unit which does not require a large space and a high cost can be readily obtained.
Also, according to the above intermediate transfer unit, since an additive with a relatively large particle diameter is added, the additive is not embedded in a mother particle for a long term but the fluidity is maintained and the quality of an image is stable. Further, since an additive with a relatively small particle diameter is also added, the surface coverage is large compared with the added weight and satisfactory transfer onto a recording medium having a rough surface is enabled.
Also, according to the above intermediate transfer unit, the occurrence of irregular color due to the transfer failure of toner of a layer farthest from a recording medium is reduced by forming a toner layer in any density area under the condition that the quantity of toner to be transferred secondarily is 1.5 mg/cm2 or less.
According to a sixth aspect of the invention, an intermediate transfer unit is provided with an intermediate transfer belt to which a toner image formed on a photoconductive drum is primarily transferred in a primary transfer position and which further secondarily transfers the toner image onto a recording medium in a secondary transfer position; primary transfer means arranged inside the intermediate transfer belt, the intermediate transfer belt being carried between the photoconductive drum and the primary transfer means in the primary transfer position; and backup means arranged inside the intermediate transfer belt and secondary transfer means arranged outside the intermediate transfer belt, the intermediate transfer belt being carried between the backup means and the secondary transfer means in the secondary transfer position, and is characterized in that the loose apparent density of the toner is set to 0.35 g/cc or more, the shape factor SF-1 of the toner is set to 150 or less, and SF-2 is set to 140 or less.
According to the intermediate transfer unit of the sixth aspect, a void is prevented from occurring in transfer by pressing the primary transfer means and the secondary transfer means onto the intermediate transfer belt in the respective transfer positions, and satisfactory transfer is enabled, even for a recording medium having an extremely irregular surface, such as recycled paper and bond paper.
According to a seventh aspect of the invention, an intermediate transfer unit is provided with an intermediate transfer belt to which a toner image formed on a photoconductive drum is primarily transferred in a primary transfer position and which further secondarily transfers the toner image onto a recording medium in a secondary transfer position, primary transfer means arranged inside the intermediate transfer belt, and secondary transfer means arranged outside the intermediate transfer belt, and is characterized in that the load of the secondary transfer position is larger than a load in the primary transfer position.
In the intermediate transfer unit of the seventh aspect, the ratio of the load in the secondary transfer position to the load in the primary transfer position is 1.5 or more.
According to the intermediate transfer unit of the seventh aspect, a void is prevented from occurring in transfer by pressing the primary transfer means on the intermediate transfer belt by a relatively small load. Satisfactory transfer is also enabled for a recording medium having an extremely irregular surface, such as recycled paper and bond paper, by pressing the secondary transfer means onto the intermediate transfer belt by a relatively large load. Further, the durability of the intermediate transfer belt can be enhanced.
According to an eighth aspect of the invention, an intermediate transfer unit is provided with an intermediate transfer belt for primarily transferring a toner image formed on a photoconductive drum and further, secondarily transferring the toner image onto a recording medium, primary transfer means arranged inside the intermediate transfer belt, and secondary transfer means arranged outside the intermediate transfer belt, and is characterized in that the hardness of the secondary transfer means is higher than that of the primary transfer means.
In the intermediate transfer unit of the eighth aspect, the hardness of the secondary transfer means is higher than that of the primary transfer means by 10 degrees or more, as measured by an Asker-C hardness meter.
According to the intermediate transfer unit of the eighth aspect of the invention, since the hardness of the primary transfer means is relatively low, a void is prevented from occurring in transfer. Since the hardness of the secondary transfer means is relatively high, satisfactory transfer is enabled for a recording medium having an extremely irregular surface and further, the turbulence of an image caused by switching the position of the secondary transfer means between positions in contact and not in contact with the intermediate transfer belt can be prevented.
According to a ninth aspect of the invention, an intermediate transfer unit is characterized in that a toner image formed on the photoconductive drum is primarily transferred onto an intermediate transfer belt by supplying bias from a high-voltage power source to a primary transfer member arranged at the rear of the intermediate transfer belt, the resistance of the primary transfer member is set to 106 to 108xcexa9, the surface resistivity of the intermediate transfer belt is set to 108 to 1012xcexa9, the volume resistivity is set to 108 to 1012 xcexa9cm, the high-voltage power source has constant-current control when impedance in the primary transfer part is large and has constant-voltage control when the impedance is small.
According to the intermediate transfer unit of the ninth aspect of the invention, the control of the high-voltage power source is optimized. Therefore, since control under fixed current is executed in the case of a printing pattern in which 2 to 4 toner layers are overlapped, that is, when impedance is large, a required transfer electric field is secured every toner layer. In the meantime, since control under fixed voltage is executed in the case of a pattern in which the ratio of printing is small, that is, when impedance is small, a required and minimum electric field for transferring toner is secured. Also, since the resistance of the primary transfer member and the intermediate transfer belt is optimized, transfer is enabled at required and minimum voltage and current, and an imperfect image caused, for example, by abnormal discharge, can be prevented.
Also, since the hardness of the primary transfer member and a load onto the photoconductive drum by the primary transfer member are optimized, the dislocation of an image in primary transfer is prevented and a void can be prevented from occurring.
Also, a void can be prevented by optimizing both the quantity of an additive having a small particle diameter and the quantity of an additive having a large particle diameter. The two types of additives different in a particle diameter added to toner secure the fluidity of the toner and inhibit the deterioration of density due to aging.
According to a tenth aspect of the invention, an intermediate transfer unit is characterized in that a toner image formed on a photoconductive drum is primarily transferred onto an intermediate transfer belt, the toner image is secondarily transferred onto a recording medium by supplying bias from a high-voltage power source to a secondary transfer member pressed onto the backup roller, the resistance of the secondary transfer member is set to 106 to 108xcexa9, the surface resistivity of the intermediate transfer belt is set to 108 to 1012xcexa9, the volume resistivity is set to 108 to 1012 xcexa9cm, the high-voltage power source has constant-current control when impedance in the secondary transfer part is large and has constant-voltage control when the impedance is small.
According to the intermediate transfer unit of the tenth aspect of the invention, the control of the high-voltage power source is optimized. Therefore, when impedance is large, as in transferring onto a recording medium in environment in which temperature and humidity are low onto an OHP sheet, a transfer electric field required for constant-current control is secured and high transfer efficiency is maintained. In the meantime, since constant-voltage control is executed when impedance is small, such as in transferring onto a recording medium in a high temperature and humidity environment and where a width of a recording medium is narrower than that of the secondary transfer member, a required and minimum electric field for transferring toner is secured. Also, since the resistance of the secondary transfer member and the intermediate transfer belt is optimized, transfer is enabled at required and minimum voltage and current, thus preventing an imperfect image due to, for example, abnormal discharge.
Also, since the hardness of the secondary transfer member and a load onto the backup roller by the secondary transfer member are optimized, the dislocation of an image in secondary transfer is prevented and satisfactory transfer is also enabled onto a recording medium having a rough surface, such as bond paper.
Also, a void can be prevented from occurring by optimizing both the quantity of an additive with a small particle diameter and the quantity of an additive having a large particle diameter. The two types of additives different in a particle diameter added to the toner secure the fluidity of the toner and inhibit the deterioration of density due to aging.
According to an eleventh aspect of the invention, an intermediate transfer unit for primarily transferring a toner image formed on a photoconductive drum onto an intermediate transfer belt by supplying bias from a high-voltage power source to a primary transfer member arranged at the rear of the intermediate transfer belt and secondarily transferring the toner image onto a recording medium by supplying bias from a high-voltage power source to a secondary transfer member pressed on a backup roller, is characterized in that the primary transfer member and the secondary transfer member are formed by an elastic body, and the variation of the resistance of the secondary transfer member due to environment is set so that it is larger than that of the primary transfer member.
According to the intermediate transfer unit of the eleventh aspect of the invention, the change of the resistance of the primary transfer member and the secondary transfer member due to environment is optimized. Since the primary transfer member is made of a member having small change of resistance due to the environment, the required capacity of a primary transfer power source can be reduced. In the meantime, since the secondary transfer member is made of a member having large change of resistance due to the environment, no failure of transfer occurs in either of a low temperature and low humidity environment or in a high temperature and high humidity environment because the resistance changes approximately to that of a recording medium, such as paper.
According to a twelfth aspect of the invention, an intermediate transfer unit primarily transfers a toner image formed onto a photoconductive drum onto an intermediate transfer belt by applying bias from a high-voltage power source to a primary transfer member arranged in a position different from a primary transfer part on the surface of the intermediate transfer belt, and secondarily transfers the toner image onto a recording medium by applying bias to a secondary transfer member, and is characterized in that a backup member in the primary transfer part is an elastic body, the resistance of the primary transfer member is set to 1 Mxcexa9 or less, and a high-voltage power source for applying bias to the primary transfer member has current absorbable constant-voltage control.
According to a thirteenth aspect of the invention, an intermediate transfer unit primarily transfers a toner image formed on a photoconductive drum onto an intermediate transfer belt by applying bias from a high-voltage power source to a primary transfer member arranged in a position different from a primary transfer part on the surface of the intermediate transfer belt, and secondarily transfers the toner image onto a recording medium by applying bias to a secondary transfer member, and is characterized in that a backup member in the primary transfer part is an elastic body, the resistance of the primary transfer member is set to 1 Mxcexa9 or less, and a resistor is connected to a high-voltage power source, which applies bias to the primary transfer member, in parallel.